Threshold templating for digital AGC

Abstract

A system and method automatically adjusts a sensing threshold in a cardioverter/defibrillator which receives electrical activity of the heart and delivers shock pulses in response thereto. An amplifier amplifies the electrical activity. A detection circuit detects depolarizations in the amplified electrical activity and provides a detect signal representing a cardiac event indicative of a depolarization when the amplified electrical activity exceeds a variable sensing threshold. Digital template generation circuitry responds quickly to track amplitudes of the electrical activity and to set the variable sensing threshold to a level proportional to a peak value of the amplitude of the amplified electrical activity and then decreases the variable sensing threshold from the level in discrete steps until the variable sensing threshold is at a low threshold value. The discrete steps are grouped into step groups. Each step group decreases the variable sensing threshold by a defined percentage.

What we claim is: 
     
       1. A method for automatically adjusting a sensing threshold in a cardioverter/defibrillator, which receives electrical activity of the heart and provides shock pulses in response thereto, the method comprising the steps of: amplifying the electrical activity of the heart;   detecting cardiac events representing depolarizations in the electrical activity which exceed a variable sensing threshold;   tracking amplitudes of the amplified electrical activity;   adjusting the variable sensing threshold to a level proportional to the amplitude of the amplified electrical activity of a current detected cardiac event; and   decreasing the variable sensing threshold from said level in discrete steps until the variable sensing threshold is at a low threshold value, wherein said discrete steps are grouped into step groups, wherein each step group decreases the variable sensing threshold by a defined percentage, and wherein each step group comprises multiple discrete steps.   
     
     
       2. The method of claim 1 wherein the adjusting and decreasing steps achieve a piecewise linear approximation of a geometric progression. 
     
     
       3. The method of claim 2 wherein the geometric progression is an exponential decay curve. 
     
     
       4. The method of claim 1 wherein said level is a percentage of a peak value of the amplitude of the amplified electrical activity of the current detected cardiac event. 
     
     
       5. The method of claim 4 wherein the percentage of the peak value is approximately 75%. 
     
     
       6. The method of claim 1 wherein the step of adjusting the variable sensing threshold to said level occurs prior to the end of a new sensed refractory period caused by a cardiac event. 
     
     
       7. The method of claim 1 wherein the method automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the step of adjusting the variable sensing threshold to said level occurs at the end of a paced/shock refractory period resulting from a pace or shock pulse. 
     
     
       8. The method of claim 1 wherein the defined percentage is approximately 50%. 
     
     
       9. The method of claim 1 wherein each step group includes at least 4 discrete steps. 
     
     
       10. The method of claim 1 wherein the decreasing step uses integer math to calculate an amount of drop for a discrete step. 
     
     
       11. The method of claim 1 further comprising the step of varying a time width of each discrete step based on operating conditions of the cardioverter/defibrillator to control the decay rate of the variable sensing threshold. 
     
     
       12. The method of claim 11 wherein the method automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the step of varying the time width of each discrete step includes the steps of varying time widths based on bradycardia pacing, tachyarrhythmia sensing, and normal sinus sensing to control the decay rate of the variable sensing threshold. 
     
     
       13. The method of claim 12 wherein the varying step varies the time width as a function of a bradycardia pacing rate when the cardioverter/defibrillator is operating under the bradycardia pacing conditions. 
     
     
       14. The method of claim 1 wherein the method automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the method further comprises the steps of: setting the variable sensing threshold after a pace or a shock pulse to a selected relatively high threshold value; and   holding the variable sensing threshold at the selected relatively high threshold value through a paced/shock refractory period resulting from the pace or shock pulse.   
     
     
       15. A system for automatically adjusting a sensing threshold in a cardioverter/defibrillator, which receives electrical activity of the heart and provides shock pulses in response thereto, the system comprising: an amplifier for amplifying the electrical activity of the heart;   a cardiac depolarization detector for detecting depolarizations in the amplified electrical activity of the heart and providing a detect signal representing a cardiac event indicative of a depolarization when the amplified electrical activity exceeds a variable sensing threshold; and   threshold controller for tracking amplitudes of the amplified electrical activity and for adjusting the variable sensing threshold to a level proportional to the amplitude of the amplified electrical activity of a current detected cardiac event and for decreasing the variable sensing threshold from said level in discrete steps until the variable sensing threshold is at a low threshold value, wherein said discrete steps are grouped into step groups, wherein each step group decreases the variable sensing threshold by a defined percentage, and wherein each step group comprises multiple discrete steps.   
     
     
       16. The system of claim 15 wherein the threshold controller adjusts the variable sensing threshold to achieve a piecewise linear approximation of a geometric progression. 
     
     
       17. The system of claim 16 wherein the geometric progression is an exponential decay curve. 
     
     
       18. The system of claim 15 wherein said level is a percentage of a peak value of the amplitude of the amplified electrical activity of the current detected cardiac event. 
     
     
       19. The system of claim 18 wherein the percentage of the peak value is approximately 75%. 
     
     
       20. The system of claim 15 wherein the threshold controller adjusts the variable sensing threshold to said level prior to the end of a new sensed refractory period caused by a cardiac event. 
     
     
       21. The system of claim 15 wherein the cardioverter/defibrillator, in which the system of claim 15 automatically adjusts the sensing threshold, has pacing capability, and wherein the system further comprises means for detecting the end of a paced/shock refractory period resulting from a pace or shock pulse from the cardioverter/defibrillator and, and wherein the threshold controller includes means for adjusting the variable sensing threshold to said level at the detected end of a paced/shock refractory period. 
     
     
       22. The system of claim 15 wherein the defined percentage is approximately 50%. 
     
     
       23. The system of claim 15 wherein each step group includes at least 4 discrete steps. 
     
     
       24. The system of claim 15 wherein the threshold controller calculates an amount of drop for a discrete step using integer math. 
     
     
       25. The system of claim 15 further comprising a decay rate controller for varying a time width of each discrete step based on operating conditions of the cardioverter/defibrillator to control the decay rate of the variable sensing threshold. 
     
     
       26. The system of claim 25 wherein the cardioverter/defibrillator, in which the system of claim 25 automatically adjusts the sensing threshold, has pacing capability, and wherein the decay rate controller varies the time widths of each step based on bradycardia pacing, tachyarrhythmia sensing, and normal sinus sensing. 
     
     
       27. The system of claim 26 wherein the decay rate controller varies the time width as a function of a bradycardia pacing rate when the cardioverter/defibrillator is operating under the bradycardia pacing conditions. 
     
     
       28. The system of claim 15 wherein the system automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the system further comprises: means for setting the variable sensing threshold after a pace or a shock pulse to a selected relatively high threshold value and for holding the variable sensing threshold at the selected relatively high threshold value through a paced/shock refractory period resulting from the pace or shock pulse.   
     
     
       29. A method for automatically controlling a decay rate of a sensing threshold in a cardioverter/defibrillator with pacing capability, which receives electrical activity of the heart and provides shock pulses and pacing pulses in response thereto, the method comprising the steps of: amplifying the electrical activity of the heart;   detecting cardiac events representing depolarizations in the electrical activity which exceed a variable sensing threshold;   tracking amplitudes of the amplified electrical activity;   adjusting the variable sensing threshold to a level proportional to the amplitude of the amplified electrical activity of a current detected cardiac event;   decreasing the variable sensing threshold from said level in discrete steps until the variable sensing threshold is at a low threshold value; and   controlling the decay rate of the variable sensing threshold by varying a time width of each discrete step based on operating conditions of the cardioverter/defibrillator including bradycardia pacing, tachyarrhythmia sensing, and normal sinus sensing.   
     
     
       30. The method of claim 29 wherein the controlling step varies the time width as a function of a bradycardia pacing rate when the cardioverter/defibrillator is operating under bradycardia pacing conditions. 
     
     
       31. The method of claim 30 wherein the controlling step varies the time width under bradycardia pacing conditions based on the time between each pacing pulse, a programmed paced refractory period, and the number of discrete steps to go from said level to the low threshold level. 
     
     
       32. The method of claim 29 wherein the controlling step varies the time width to produce a relatively fast decay rate when the cardioverter/defibrillator is operating under tachyarrhythmia sensing conditions. 
     
     
       33. A system for automatically controlling a decay rate of a sensing threshold in a cardioverter/defibrillator with pacing capability, which receives electrical activity of the heart and provides shock pulses and pacing pulses in response thereto, the system comprising: an amplifier for amplifying the electrical activity of the heart;   a cardiac depolarization detector for detecting depolarizations in the amplified electrical activity of the heart and providing a detect signal representing a cardiac event indicative of a depolarization when the amplified electrical activity exceeds a variable sensing threshold;   threshold controller for tracking amplitudes of the amplified electrical activity and for adjusting the variable sensing threshold to a level proportional to the amplitude of the amplified electrical activity of a current detected cardiac event and for decreasing the variable sensing threshold from said level in discrete steps until the variable sensing threshold is at a low threshold value; and   decay rate controller for controlling the decay rate of the variable sensing threshold by varying a time width of each discrete step based on operating conditions of the cardioverter/defibrillator including bradycardia pacing, tachyarrhythmia sensing, and normal sinus sensing.   
     
     
       34. The system of claim 33 wherein the decay rate controller varies the time width as a function of a bradycardia pacing rate when the cardioverter/defibrillator is operating under bradycardia pacing conditions. 
     
     
       35. The system of claim 34 wherein the decay rate controller varies the time width under bradycardia pacing conditions based on the time between each pacing pulse, a programmed paced refractory period, and the number of discrete steps to go from said level to the low threshold value. 
     
     
       36. The system of claim 33 wherein the decay rate controller varies the time width to produce a relatively fast decay rate when the cardioverter/defibrillator is operating under tachyarrhythmia sensing conditions.